Our present invention relates to an air demoisturizer for electric power apparatus like power transformers, power chokes and other inductive equipment, tap changers and like apparatus which is oil-insulated or oil-cooled and which usually includes an oil expansion vessel or tank which can be connected to an external source of air, which must be demoisturized.
The provision of an air demoisturizer to remove moisture from the atmospheric air is supplied to an oil-expansion tank of oil-insulated power equipment is described, for example, in DE 201 04 259 U1.
The apparatus described in that German patent document comprises a receptacle containing a dehumidifying agent, i.e. an absorbent, in conjunction with an oil receiver through which the moisturized or dehumidified air is supplied to the oil-expansion tank or vessel of the power equipment.
An oil-filled transformer generally has such an oil expansion vessel in order to compensate for volume changes in the transformer oil resulting from temperature fluctuations. Since the air which is drawn into the expansion tank cannot carry any moisture into the system without decreasing the breakdown voltage of the insulating oil, the demoisturizer or dehumidifier is as a rule provided to treat the air admitted to the expansion vessel. The air which is drawn into this vessel can also be passed through an oil reservoir as noted as a prevention of contamination of this air as dried in the dehumidifier.
The basic structure of such an air demoisturizer can be found in the German Industrial Standard DIN 42 562, parts 1 and 2, and the demoisturizer absorbent can be, for example, a silica gel.
EP 0 245 598 deals generally with gas cushions for transformers and the like for compensating for the thermal expansion and contraction of the transformer oil.
EP 0 746 000 describes how the insulating oil of a transformer may be kept dry utilizing silica gel.
DE 34 13 071 describes an air demoisturizer for a transformer.
DE 34 13 888 is directed to a device for maintaining an electric insulating system free from hygroscopicity.
If one considers the art as a whole, it will become apparent that the moisture absorbing agents which have been used can be subdivided into two major groups. One of these groups includes materials like zeolites which only can take up a limited amount of water before they reach a saturation point and lose their drying properties and must be replaced. The other major group consists of materials like silica gel which has already been mentioned and the so-called KC DRYING BEADS ORANGE which only bind water physically and then can be restored to their original dry state by a heating, thereby eliminating the need to replace the material itself. In the past, however, in the application previously described, this heating was effected only following complete separation of the dehumidifier from the apparatus, dismounting of the demoisturizer and heating in accordance with German Industrial Standard DIN 42 562 in an appropriate device.
DE 201 04 259 mentioned previously describes an air demoisturizer which is equipped with control contacts which forms part of a monitoring unit for monitoring the moisture content of the moisture-absorbing medium. Instead of a replacement of the absorbent on a time basis, therefore, replacement of the absorbent occurs as need arises because of moisture build-up in the absorbent. When the monitoring device indicates that the absorbent is saturated with water, a signal for replacement of the absorbent is emitted. The replacement of the medium itself is effected as in the art and the system of this patent publication can thus provide only better utilization of the absorbent since the replacement is then matched to the occurrence of saturation.
In practice, the entire approach of replacing the absorbent is inconvenient. The connection of the expansion duct to the transformer must be closed, the entire demoisturizer must be dismounted, the individual compartments of the system must be emptied and a new fresh absorbent must be introduced. The amount of absorbent used is usually 1.2 to 4.5 kilograms and thus the entire process is expensive, inconvenient and time-consuming.
It is, therefore the principal object of the present invention to provide an improved air demoisturizer which eliminates the drawbacks of prior art demoisturizers for the purposes described.
Another object of the invention is to provide an air demoisturizer in which replacement of the absorbent is no longer required and which allows the demoisturizer to regenerate the absorbent in situ to restore its dehumidifying function after it has been saturated with water.
It is also an object of this invention to reduce the operating cost and the down time of power equipment provided with the air demoisturizing of the invention.
These objects and others which will become apparent hereafter are attained, in accordance with the invention with an air demoisturizer in which the air-permeable body of the absorbent capable of regenerating by heating from a saturated state to the absorbent, can be heated by a heater so disposed in or on the receptacle as to heat the body to regenerate the absorbent.
More particularly, the air demoisturizer for an oil-expansion vessel of an oil-insulated electrical apparatus can comprise:
a receptacle containing an air-permeable body of a moisture absorbent capable of regeneration by heating from a saturated state of the absorbent, and having an air inlet on one side of the body and an air outlet on an opposite side of the body;
an oil receiver connected to the air outlet of the receptacle and containing an oil bath through which demoisturized air from the receptacle is conducted, the oil receiver having a discharge port formed above the bath and connectable with the oil-expansion vessel of the electrical apparatus;
a monitoring unit responsive to moisture in the body for indicating a moisture content thereof; and
a heater positioned to heat the body in the receptacle to regenerate the absorbent.
The oil receiver advantageously comprises a riser extending through the oil sump and communicating with the air outlet, and a bell over the riser and reaching downwardly into the bath.
The heater can be an electrical resistance heater which can include individual heating elements received in the body and surrounded by absorbent, i.e. heating elements which are directly in contact with the absorbent.
The invention thus utilizes a drying agent or absorption agent which is provided with an electric heater and which can be heated to regenerate that absorbent in situ, the heater being controlled in response to the moisture-content state of the absorption agent or as a function of time. The absorbent can be heated and dried upon reaching the moisture-saturation point and thereby is returned to an operable state for absorbing moisture. The absorption agent can be an air-permeable solid body or a mass of particles, e.g. granules, in which the heater is embedded and the liquid liberated by heating can be discharged downwardly, e.g. as droplets. As a consequence the oil receiver is preferably located above the receptacle containing the absorbent. The apparatus can be provided with a temperature monitoring element which can cut off the heating when the temperature in the absorbent exceeds a limiting temperature to prevent over-heating.
The electrical connection and contact elements for the heater can be provided in a switch housing which can be disposed laterally of the demoisturizer. The moisture sensor which is normally provided to monitor the moisture content of the air emerging from the demoisturizer can also be mounted in or on this switch housing and can have its control circuitry built into the latter.
The magnetic valve can be provided between the receptacle and the oil receiver or between the oil receiver and the oil-expansion tank to cut off flow of air to the oil-expansion tank during the operation of the heater to regenerate the absorbent.
The electric heating can be effected in various ways. For example, it can be turned on (once the magnetic valve is closed) when the moisture sensor detects saturation of the demoisturizer, i.e. when the moisture content of the air about to be drawn into the oil-expansion tank, triggers a signal that the moisture content is impermissibly high. The heating element is then turned on as a function of the state of saturation of the absorbent.
The heater can also be turned on cyclically, e.g. every 48 hours, in a time-dependent operation. Both methods can be combined. It is however also possible to operate the heating based upon external information, such as by means of a transformer-monitoring system, or example, using xe2x80x9cTransformer Management System TMS(copyright)xe2x80x9d of the assignee of this application.